RECHARGE AT YUCCA MOUNTAIN, SOUTHERN NEVADA AND BLACK MESA, ARIZONA: ESTIMATES FROM THE CHLORIDE MASS BALANCE METHOD AND CHLORINE-36 DATA

Average Holocene and late Pleistocene recharge rates at Yucca Mountain, southern Nevada, and at Black Mesa, Arizona, are estimated from the chloride mass balance method. Yucca Mountain is being evaluated as a potential site for a geologic repository for high-level nuclear waste. The amount of local recharge is important to the proposed repository because it is water that may pass through the repository. At Black Mesa, Arizona, coal mining raises concerns for depleting the only source of drinking water in the region. For water resource evaluation, the availability of groundwater resources is in part determined by the recharge into aquifers. Accurate estimates of recharge to aquifers in arid and semiarid areas is a challenge because recharge fluxes are low and spatially and temporally variable.

In this study, we first derived Holocene and late Pleistocene Cl^- deposition rates from ³⁶Cl deposition rates and ³⁶Cl/Cl ratios in groundwater and packrat middens. Calculated Cl^- deposition rates were lower in late Pleistocene than Holocene at Yucca Mountain but higher in late Pleistocene than Holocene at Black Mesa. Recharge rates were then calculated from Holocene and late Pleistocene Cl^- deposition rates, respectively. The calculated average Holocene recharge at Black Mesa is 9 mm/yr, and the average recharge in late Pleistocene is 35 mm/yr. The temporal variation patterns of recharge compare well with those estimated independently from numerical models calibrated to the distribution of radiocarbon dates and with known climate changes in the area. Local recharge rates at Yucca Mountain were estimated from the ³⁶Cl/Cl ratios and Cl^- concentrations in perched waters. Estimated recharge for late Pleistocene is about 15 mm/yr and for Holocene is 5 mm/yr. Although there is uncertainty in these estimates, greater confidence can be placed in the relative rates of recharge estimated for the late Pleistocene and the Holocene. These estimates agree well with spatially and time-averaged net infiltration estimates for present-day and glacial-transition climates (4.6 mm/yr and 15.6 mm/yr, respectively) obtained from a watershed-scale infiltration model of Yucca Mountain. [This work, performed in part under U.S. Nuclear Regulatory Commission (NRC) contract NRC-02-97-009, does not necessarily reflect views or position of the NRC.]